1. Field of the Invention
The present invention relates to capacitors consisting of ceramic electronic components to be mounted on circuit boards and, more particularly, relates to a cooling structure of a snubber capacitor and an inverter device using the cooling structure of the capacitor.
2. Description of the Related Art
In recent years, motors equipped with inverters have become widespread from the flow of energy saving. FIG. 6 is an example of a circuit diagram of a three phase inverter which is for driving a motor 12. As shown in the drawing, a plurality of semiconductor switching elements 2 (when named generically, additional characters will not be repeated later), each of the plurality of semiconductor switching elements 2 consisting of a corresponding one of semiconductor switching elements 2a, 2c, and 2e on the upper arm side and a corresponding one of semiconductor switching elements 2b, 2d, and 2f on the lower arm side, are included in each of U phase, V phase, and W phase; and the semiconductor switching elements 2 are connected to a power source 13 via wiring inductances 11a. Furthermore, a snubber capacitor 1 is connected in parallel between the respective semiconductor switching elements of the upper and lower arms via wiring inductances 11b. 
The aforementioned snubber capacitor 1 is used for suppressing a surge voltage generated in switching the semiconductor switching elements 2. The surge voltage ΔV is found by an equation ΔV=−L(di/dt) when a main circuit inductance of the inverter device is set to “L.” When the surge voltage increases, capacitor capacitance required for obtaining a sufficient effect of surge elimination increases; and accordingly, the whole of the inverter device becomes larger in size.
In order to reduce the main circuit inductance, a parasitic inductance that arises in association with an external wiring which connects a smoothing capacitor to the semiconductor switching element and an internal wiring which connects the snubber capacitor to the semiconductor switching element needs to be reduced; and therefore, it is preferable to arrange the snubber capacitor 1 in the vicinity of the semiconductor switching element 2 to reduce the parasitic inductance.
Furthermore, in recent years, the switching speed of the semiconductor switching element is required to be increased; and in the aforementioned snubber capacitor 1, a capacitor with low impedance (equivalent series resistance (ESR)) at a high frequency is preferable. In addition, an impedance (ESR) component contained in the capacitor causes heat generation; and therefore, the capacitor with low impedance (ESR) is preferable.
From such a background, a laminated ceramic capacitor which is low impedance (ESR) at the high frequency side and is capable of being arranged in the vicinity of the semiconductor switching element has been widely used as the snubber capacitor 1.
However, as in the inverter device for use in an automobile, mechanical stress is generated due to the difference of coefficients of thermal expansion between the laminated ceramic capacitor and a circuit board to which the laminated ceramic capacitor is mounted in circumstances whose temperature change and temperature cycle are large; and accordingly, a crack may be generated in the laminated ceramic capacitor.
More particularly, in the case where a metal circuit board serving as a high heat dissipation substrate has a large coefficient of thermal expansion and the laminated ceramic capacitor is mounted on the metal circuit board, a problem exists in that the occurrence of the crack caused by the difference of the coefficients of thermal expansion becomes prominent and accordingly reliability of the capacitor cannot be sufficiently kept.
Consequently, for example, a laminated ceramic capacitor 1a is used as the capacitor 1 as shown in FIG. 7. The laminated ceramic capacitor 1a in which lead terminals 103 formed by processing a metal plate are attached to external electrodes 102 of a ceramic electronic component element 101 is mounted on a circuit board or the like via the lead terminals 103; and accordingly, mechanical stress caused by the difference of coefficients of thermal expansion between the ceramic electronic component element 101 and the circuit board is reduced.
Furthermore, in a laminated ceramic capacitor 1b used as the capacitor 1 shown in FIG. 8, lead terminals 103 are attached to metal plates 104 joined to external electrodes 102 by applying conductive paste to the external electrodes 102 on both ends of a ceramic electronic component element 101; and accordingly, the laminated ceramic capacitor 1b in which the joining strength of the lead terminals 103 is excellent and reliability is high can be obtained. (See Patent Document 1.)
In addition, in a laminated ceramic capacitor 1c used as the capacitor 1 shown in FIG. 9, material 107 having a thermal expansion coefficient, which is larger than a thermal expansion coefficient of a ceramic electronic component element 101 and smaller than a thermal expansion coefficient of a metal substrate of aluminum, is made to intervene between the ceramic electronic component element 101 and the metal substrate (not shown in the drawing) whose base plate is made of aluminum, and electrically-conducting paths 106 being in contact with external electrodes 102 of the ceramic electronic component element 101 are joined by solder 105; and accordingly, the laminated ceramic capacitor 1c which prevents the ceramic electronic component element 101 from being cracked and has high reliability can be obtained. (See Patent Document 2.)
Further, a cooling structure of a snubber capacitor to be connected in parallel to semiconductor switching elements of an inverter circuit is configured such that the snubber capacitor is placed in a module filled with mold material in order to prevent insulation deterioration due to moisture absorption and an electrode of the snubber capacitor is connected to a conductor connected to a cooling fin for cooling semiconductor switching elements via a sheet having high insulation property and high thermal conductivity. (See Patent Document 3.)
[Patent Document]
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-306764
[Patent Document 2] Japanese Unexamined Patent Publication No. H06-077631
[Patent Document 3] Japanese Unexamined Patent Publication No. 2008-113511
By the way, in an inverter device having a large current of several hundred amperes and high speed switching, a peak current generated by a surge voltage is large and self-heating of a snubber capacitor to which the peak current flows cannot be negligible.
In the inverter device for large current as described above, the heat of the semiconductor switches, the snubber capacitors, and the like mounted on the circuit board is dissipated by using the metal substrate serving as the high heat dissipation substrate and the circuit board of a ceramic substrate. However, in the case where the conventional laminated ceramic capacitors 1a, 1b, and 1c shown in FIG. 7 to FIG. 9, whose mechanical stresses are reduced, are mounted as the snubber capacitors, paths through which the heat is dissipated to the metal substrate are limited to the lead terminals and the electrically-conducting paths. If the coefficient of thermal conductivity and the thickness of material for use in the lead terminal and the electrically-conducting path are not sufficient, the heat generated in the snubber capacitor is not sufficiently dissipated to the substrate side; and accordingly, the temperature of the laminated ceramic capacitor rises. Therefore, a problem exists in that the temperature rise of the capacitor shortens operating life of the capacitor and reduces reliability thereof.
Furthermore, the cooling structure of the snubber capacitor shown in Patent Document 3 uses the sheet having high insulation property and high thermal conductivity; however, the sheet intervenes only between the electrode of the snubber capacitor and the conductor connected to the cooling fin. The paths through which the heat is dissipated to the cooling fin are originally limited to the capacitor electrode and the conductor; and therefore, a problem exists in that if the coefficients of thermal conductivity of the material for use in the capacitor electrode and the conductor are not sufficient, the heat generated by the snubber capacitor is not still dissipated sufficiently to the cooling fin side and accordingly the temperature of the snubber capacitor rises.